This invention relates to improvements in superconducting coils and more particularly, but not by way of limitation, to an electronic circuit for the detection and analysis of normal zones in a superconducting coil.
Certain materials have the property, at low temperatures in the neighborhood of the absolute zero point, of becoming superconducting, implying that the resistivity at a certain critcal temperature suddenly sinks to zero. The critical temperature for lead is about 7.degree. K. and for mercury 4.degree. K. This property has been utilized by using superconductors in the magnetizing winding of very large magnets with large field strength. Such magnets are suitably built without an iron circuit. The necessary magnetizing power becomes equal to zero since the superconductor has no resistance, but of course a certain amount of power is consumed in keeping the superconductor at the necessary temperature. Generally, the cooling is performed by means of liquid helium.
Problems arise, however, if for some reason an increased temperature occurs at some point on the superconductor and the superconductivity as a result ceases to exist at this point. The electro-magnetic energy will be large and the damage which may result from transition of the superconductive winding to a resistive state may be extensive. This damage may result from carbonization of insulators, fusion of conductors and deformation of mechanical supports, all of which are extensive and difficult to repair, as well as resultant unsafe conditions due to rapid evaporation of a large quantity of liquid helium or other cryogenic fluid.
The process of a portion of a superconducting coil going resistive may also become explosive since the very large magnetic energy of the magnet coil will be discharged in a short time in the non-superconducting zone, which will be very short as the heat generated by current losses in the zone will not have time to spread to other parts of the coil.
It is necessary to be able to detect transistion of any portion of a superconducting coil as soon as this transition occurs, from superconductive to resistive state. If the length affected by the transition is small, remedial measures may still be taken. The total length, however, of superconductive windings in powerful magnets may be substantial. Yet, the resistance of the short length of the superconductor, even when in the resistive state, may be very small, since the sheathing of a superconductive wire is by a very pure metal which, in turn, in the liquid helium also reveals a very small resistance.
The resistive voltage drop, that is the IR drop, to be detected upon a transition is thus very small, in the order of a millivolt or fraction of a millivolt. The self induced potential, that is the voltage drop due to self induction (L di/dt) which appears at the terminals of the winding is, however, substantial and often in the order of tens, or more volts. If the current through the super conductive coil is varied to obtain a constant field, potential drops will likewise appear across the terminals during the variations, until a stead state is again obtained.
It is thus necessary to detect the resistive voltage drop in the order of a minor fraction of a millivolt, which might indicate a change of a portion of the superconductor to resistive state, entirely apart from the potentials due to self induction.
There have been efforts in the past to provide means for indicating the ceasing of superconductivity in a superconducting coil. U.S. Pat. No. 3,214,637 discloses such a device wherein a non-superconducting conductor is arranged to follow a superconductor closely but being electrically insulated therefrom. A voltage sensing device is arranged between the end points of the superconductor and the non-superconductor so that upon the occasion of a fault in the superconductor a voltage difference exists between these two end points and the voltage sensing device then operates to break the current in the superconductor. As noted this device requires an additional parallel coil in the close association to the superconductor and senses voltage to determine a quench condition.
U.S. Pat. No. 3,579,035 discloses the location of an auxiliary winding adjacent to the main winding of a superconductive coil with the flux relationship between the flux generated by the main winding and by the auxiliary winding being a predetermined factor. Upon transition from a superconductive state to resistant state of any portion of the main winding, the IR drop in the superconductive winding will appear as a signal between the superconductive winding and the auxiliary winding, irrespective of inductive voltages. This signal is compared with a voltage drop across a resistance in the supply of the main winding to obtain an alarm which can be used to disconnect the superconductive coil from a power supply. Again an additional coil is provided to generate voltage differences between the superconducting coil and an auxiliary or pick up coil with no attempt to measure instantaneous power.